1. Field of the Invention
The invention relates to a circuit arrangement for scanning optical recording carriers.
2. Description of Related Art
In such a circuit arrangement, data which are stored in information tracks on the information plane of the optical recording carrier are accessed by a scanning device of a laser light source, optics and a group of photodetectors. A scanning light beam emitted by the light source is reflected by the information plane and directed through the optics onto the photodetectors. By evaluating the signals of the individual photodetectors, information signals are obtained and supplied to subsequent signal processing circuits and devices. The information signals can be audio, video or data signals.
To achieve a correct reproduction of the information signals, both accurate focusing of the light beam directed onto the information plane of the recording carrier and precise guidance of the light beam along the information tracks is required. Apart from a coarse drive which can be constructed, for example, as spindle in the case of a disc-shaped recording carrier, there is precision tracking by the scanning device in that actuators preferably constructed as electromagnetic microdrives mechanically move its optics. The optics are moved along their optical axis for focusing and shifted perpendicularly to the optical axis or tilted by a small angle for tracking.
The scanning device tracking is initiated by focus error signals and track error signals which are obtained by further evaluation of the signals of the individual photodetectors. The scanning device, the actuators and additionally provided control amplifiers form control loops for the guidance of the scanning device, namely a first control loop for the focus correction and a second control loop for the track correction.
When the control loops operate optimally, the scanning device is always displaced in such a manner that its deviation from the ideal position is just compensated for. This operating mode of the control loops is significantly determined by the gains in the overall control loops, the control gains. To compensate for tolerances of control loop elements, gain controllers are provided by means of which the control gain is set during calibration and adjustment measures during the manufacture of the device.
It has been found in practice that in spite of careful setting of the gain controllers, correct reproduction of the information signals is no longer possible if the circuit arrangement is to be operated under other environmental conditions, particularly other temperatures, than prevailed during the calibration of the control gain. This state can occur even if a device with such a circuit arrangement, for example a CD player, is to be used in the colder season in unheated rooms, or in the other case is located in a particularly hot environment and in addition is exposed to the heat dissipation of other loads. Especially extreme conditions also occur when a device is operated in motor vehicles.